This invention relates to a semiconductor device and its fabrication; and more particularly to a hyperabrupt junction tuning diode and a method of fabrication which results in a tighter range of process control and improved electrical characteristics.
A tuning diode is a semiconductor device used as a voltage variable capacitor. The capacitance of the devices varies as the reverse bias voltage across the device is changed according to the equation: EQU dc/dv = C.sup.3 /KN(x)
where K is a constant
Circuit applications of tuning diodes require that the capacitance vary with voltage in a prescribed manner and that this capacitance-voltage response be reproducible from device to device. The capacitance-voltage response is governed by the term N(x) in the above equation where N(x) is the doping density in the bulk of the tuning diode as a function of the distance x away from the diode junction. For conventional diodes the capacitance varies approximately as the square root of the voltage for shallow step junctions and approximately as the cube root of the voltage for deeper, linearly graded junctions. For tuning diodes the capacitance response is often required to be a stronger function of the voltage than the cube root or square root response of conventional diodes. This requires a particular tailoring of the doping distribution, N(x), for the fabrication of a tuning diode with a given capacitance response. This has been accomplished, for example, by fabricating the tuning diode in an epitaxial layer of silicon having a carefully retrograded impurity profile. (Heidenreich, U.S. Pat. No. 3,523,838.) Control of the characteristics of the diode then becomes the problem of controlling a very difficult epitaxial layer deposition process.